Two-bed PSA oxygen processes incorporate a vacuum step that is used to desorb nitrogen from the molecular sieve. During this vacuum step a Roots-type rotary blower is used. As the gas passes across the blower from a region of relatively low pressure to a region of relatively high pressure it is compressed, and consequently heated.
Table 1 illustrates typical heat of compression values (measured as .DELTA.T.sub.rise =blower discharge temperature-blower inlet temperature) associated with steady state operation of a standard rotary blower. By the term "standard rotary blower" we mean a blower that is not capable of being cooled via external means. An example of such a blower is the RAS-J Whispair, available from Dresser Industries, Connorsville, Ind.
The data in Table I below is measured at an ambient pressure of 14.5 psia and ambient temperature of 70.degree. F. The table represents steady state conditions.
TABLE 1 Performance Data for a Rotary Positive Blower having no external cooling .DELTA.P = Inlet P - Outlet P (PSIA) .DELTA.T.sub.rise 0 7 0.5 13 1 19 1.5 26 2 33 2.5 42 3 51 3.5 61 4 72 4.5 84 5 98 5.5 113 6 131 6.5 151 7 175 7.5 202 8 234 8.5 272 9 318 9.5 375 10 447
As can be seen, as the difference between the pressure at the inlet of the blower and the outlet of the blower (.DELTA.P) increases, the heat of compression temperature at the outlet of the blower increases. This increase in temperature (.DELTA.T.sub.rise) leads to the heating of the compressor components which, if excessive, result in breakdown of the compressor. Thus if a Roots-type blower has a rated temperature tolerance (.DELTA.T.sub.max) of, for example, 230.degree. F., this steady state .DELTA.T value is exceeded at a steady state .DELTA.P of about 8.
Unfortunately, typical PSA systems having one or more adsorbent beds may operate with .DELTA.P values that are greater than about 8. Such systems are described in U.S. Pat. No. 5,658,371 (single bed), U.S. Pat. No. 5,518,526 (multi-bed) and in co-pending application Ser. No. 08/611,942 (multi-bed), all of the above being incorporated herein by reference. Typical .DELTA.P values for the cycles disclosed in the above patents range from about 6 to 12 psia, preferably 7-11 psia, most preferably 8-10 psia.
Within the above ranges, the .DELTA.P values for a two-bed system are greater than those for a single-bed system.
The problem of excessive .DELTA.T has been addressed through the use of air or water cooled rotary blowers (e.g. blowers comprising additional means such that the compressor may be cooled via the use of external fluids) to lower the gas discharge temperature. Air cooled blowers are typically used with single bed systems, while water cooled blowers have been necessary for multi-bed systems. Examples of air cooled blowers may be found in U.S. Pat. Nos. 5,090,879 and 4,859,158. A commercially available blower of this type is the Roots DVJ Dry Vacuum Whispair Blower, available from Dresser Industries, Inc. in Connorsville, Ind. Other methods of cooling rotary blowers include the use of heat radiating fins, heat sinks and the like.
.DELTA.P and steady state .DELTA.T.sub.rise values for a typical air cooled rotary blower are illustrated in Table 2. The data in Table II below is measured at an ambient pressure of 15 psia and ambient temperature of 70.degree. F. This table represents steady state conditions.
TABLE 2 Performance Data for a Cooled Rotary Positive Blower .DELTA.P = Inlet P - Outlet P (PSIA) .DELTA.T.sub.rise 0.5 5.2 0.98 10.6 1.46 17.8 1.95 23.2 2.43 30.4 2.91 39.4 3.4 48.4 3.88 57.4 4.36 66.4 4.85 77.2 5.33 89.8 5.82 100.6 6.3 113.2 6.78 125.8 7.75 152.8 8.23 167.2 8.72 181.6 9.2 196 9.68 210.4 10.17 226.6
As is apparent from these values, a cooled blower allows for operation at greater .DELTA.P than a non-cooled blower; and .DELTA.T.sub.max of 230.degree. F. is not exceeded until .DELTA.P of greater than about 10 is achieved. Thus it is apparent from the available data that if one wished to operate a process having a .DELTA.P between about 8 to 10, then a cooled blower would be presumed to be required.
Unfortunately, such blowers have higher operating and capital costs. In addition, water cooled blowers require the availability of water to inject into the blower housing and in many cases water is not readily available. In addition, the cooling water causes scale to form on the blower end plates and rotors which cause mechanical problems for the blower.